As is now well known, the CT scanner produces narrow beams of radiation, typically x-radiation, through plural coplanar paths defining a cross-sectional or tomographic view of a patient's internal organs, such as the brain or the thoracic region. The attenuated beams are sensed by radiation detectors whose electrical output is indicative of the intensity of the radiation received by the detector. The electrical output of each of these detectors is passed to a signal processing channel and the data acquired by these channels is reconstructed and eventually displayed.
During the approximate 10 years of their existence, CT scanners have undergone several generations of change, each of which has been accompanied by a marked increase in the number of detectors. The architecture in use today which makes use of the greatest number of detectors employs a rotating fan beam source with a stationary arc of uniformly spaced detectors about the center point of a scan circle. The fan beam source revolves about the center point inside the detector array irradiating the scan circle and subtending at any given time only a fraction of the detectors in the total array. If desired, the array may be a complete circle or ring. In an arrangement of the above-described type, the stationary array of spaced detectors may number several hundred or more. As the number of detectors increases, it becomes more and more cumbersome to provide each detector in the array with a respective exclusive signal processing channel.
In U.S. Pat. No. 4,220,863 to McBride, et al., a data channel multiplexing system is taught for use with a 720 detector CT scanner which requires only 180 channels of signal processing electronics. In the McBride, et al. system a number of signal processing channels--180 in one embodiment--corresponding to the maximum number of detectors subtended by the fan pattern at any time is connected by switching circuitry to receive the outputs of only the irradiated detectors. This is accomplished by providing a shift register, including a series of detector bits, initially loading a predetermined number (e.g., all "ones") into a series of consecutive bits corresponding to the maximum number of detectors subtended by the radiation fan. The shift register bits are then clocked or shifted collectively each time the source advances so that it irradiates another peripheral detector. A number of gates comprising switching circuitry corresponding to the maximum number of detectors subtended by the fan pattern is connected to receive the outputs of a respective exclusive group of nonconsecutive detectors, one and only one of which is irradiated at any given time by the radiation fan. For any given position of the source, each gate permits the passage of the output of only the one irradiated detector as determined by the state of the shift register detector bits corresponding respectively to the detectors in the group received by the particular gate.